Communication line isolator

ABSTRACT

An isolator  10  is formed with an isolation transformer  13  that blocks an abnormal voltage entering from a communication line  30  and protects a protection target device  40 . A pulse transformer  43  in the device  40  includes a primary winding  43   a  connected to a secondary winding  13   b  of the transformer  13  and a secondary winding  43   b  connected to an inner circuit. A medium tap of the primary winding  43   a  is grounded through an inner capacitance Ci of the device  40 . A parasitic capacitance Cs between the primary winding  13   a  and the secondary winding  13   b  of the transformer  13  and the inner capacitance Ci are connected in series, an abnormal voltage is divided by the capacitance Cs and the capacitance Ci, and a capacitance value of the capacitance Cs is set such that the divided voltage of the capacitance Ci is equal to or below a predetermined value.

BACKGROUND ART

1. Field of the Invention

The present invention relates to a communication line isolator (which isreferred to as, for example, a surge protective device (hereinafterreferred to as “SPD”) for LAN) arranged so as to intervene in acommunication line in order to protect protection target devicescorresponding to communication devices including an optical network unit(hereinafter referred to as “ONU”), a home gateway (hereinafter referredto as “HGW”), a personal computer (hereinafter referred to as “PC”) anda modem, which are connected to a communication line such as a localarea network (hereinafter referred to as “LAN”) line. In particular, thepresent invention relates to an isolation-type communication lineisolator having high pressure resistance performance (e.g.isolation-type LAN SPD).

2. Description of the Related Art

In order to protect communication devices corresponding to protectiontarget devices from an abnormal voltage or abnormal current such as alightning surge which enters from a power wire, communication wire orgrounding wire, the SPD is installed in each wire. SPD's are broadlyclassified into two types (i.e. current release type and isolation type)as described below.

As a current-release-type SPD in the related art, for example, there areknown a current-release-type SPD for power disclosed in Japanese PatentLaid-Open No. 2008-206263 and a current-release-type SPD forcommunication disclosed in Japanese Patent Laid-Open No. 11-341677.These current-release-type SPD's are formed with an arrester, varistorand so on, and these components are grounded. As an operation, bycausing the SPD to accept an abnormal current and release the abnormalcurrent to the ground side, the abnormal current is prevented fromentering the communication device side.

Also, as an isolation-type SPD in the related art, for example, thereare known an isolation type SPD for power disclosed in Japanese PatentLaid-Open No. 2005-151705 and an isolation type SPD for communicationdisclosed in Japanese Patent Laid-Open No. 10-64740. Theseisolation-type SPD's are formed with an isolation voltage inverter (i.e.isolation transformer), and, although there are many SPD types that aregrounded by setting an electrostatic shield to the isolationtransformer, there are SPD types that are not grounded. As an operation,by causing the isolation transformer to intervene in a line of a powerwire or communication wire, an isolation unit is formed in the line, andtherefore an abnormal current does not enter the communication deviceside as long as the isolation transformer is not destroyed.

When the isolation-type SPD is explained, an isolation transformer usedin this isolation-type SPD has a parasitic capacitance between a primarywinding and a secondary winding. Therefore, even if the primary sidecorresponding to the power wire side or the communication wire side andthe secondary side corresponding to the power device side orcommunication device side are isolated using an isolation transformer,part of an abnormal current such as a lightning surge which enters fromthe primary winding side of the isolation transformer passes to thesecondary winding side of the isolation transformer by capacitivecoupling by the parasitic capacitance.

To cope with such a pending problem, like the isolation-type SPD'sdisclosed in Japanese Patent Laid-Open No. 2005-151705 and JapanesePatent Laid-Open No. 10-64740 of the related art, it is desirable tobasically set an electrostatic shield between the primary winding sideand the secondary winding side of an isolation transformer and fastenthis electrostatic shield to the ground. By this means, it is possibleto suppress an abnormal current flowed from the primary winding side tothe secondary winding side (i.e. it is possible to extremely lower aso-called surge change ratio) and therefore it is possible to reliablyprotect a power device or communication device from an abnormal currentor abnormal voltage. Also, there is a method of preventing an abnormalcurrent or abnormal voltage from intervening in the power device side orcommunication device side by setting a release-type SPD before the firstwinding side of the isolation transformer or after the secondary windingside of the isolation transformer and fastening a ground terminal ofthis release-type SPD to the ground.

Next, an explanation is given to a LAN SPD which is an “isolation type”and “does not require grounding” in the related art.

In recent years, regarding a communication line corresponding to acommunication wire, a LAN line becomes common. Communication devicessuch as an ONU, an HGW and a PC connected to the LAN line include, forexample, a LAN connector connected to the LAN line, a pulse voltageinverter (i.e. pulse transformer) connected to this LAN connector and aLAN controller that is connected to this pulse transformer and transmitsand receives pulse signals. To protect such a communication device froman abnormal voltage or abnormal current such as a lightning surge, theLAN SPD as disclosed in Japanese Patent Laid-Open No. 2011-10085 of therelated art is also installed between the communication device and theLAN line.

For example, in a communication device installed in an average house orbuilding's office, since a ground terminal is often installed in apredetermined limited place, there are many cases where it is difficultto lay a grounding wire from the LAN SPD toward the ground terminal.Therefore, as the LAN SPD used in an average house or building's office,there are many cases where an SPD which is an “isolation type” and “doesnot require grounding” is required. This kind of SPD is disclosed in,for example, Japanese Patent Laid-Open No. 2008-136303 of the relatedart.

However, the isolation-type LAN SPD which is one of communication lineisolators in the related art has following problems (a) and (b).

(a) Regarding Cooperation Problem

A withstand voltage value of an isolation-type LAN SPD using anisolation transformer in the related art is often designed to around 5kV. This is because of considering the reality that an abnormal voltageof a lightning surge which enters a LAN line is often equal to or below5 kV. Further, when an excessive voltage of around 5 kV is applied tothe primary side of the LAN SPD, although a voltage is caused on thesecondary side of the LAN SPD by capacitive coupling of an isolationtransformer, the voltage caused on the secondary side is around 1 kV ata maximum. A communication device connected to the secondary side of theLAN SPD normally has a structure to withstand a voltage of around 1 kV.

Meanwhile, according to the latest studies and field investigationreport in recent years, a case is reported where a lightning surge ofaround 13 kV with an impulse waveform of 10/700 μs (i.e. a lightningsurge with a waveform which: reaches the maximum voltage of 13 kV at thetiming a time of 10 μs passes from the rising start timing; after that,gradually attenuates; and, at the timing a time of 700 μs passes fromthe rising start timing, attenuates up to 7.5 kV, which is a halfvoltage of the maximum voltage of 13 kV) occurs. Therefore, there is aneed to improve a withstand voltage of the LAN SPD up to around 13 kV.

To improve the withstand voltage of the LAN SPD itself, the design of anisolation transformer system (e.g. shape) has to be changed.

However, simply, in a case where the withstand voltage performance ofthe LAN SPD is improved to around 13 kV, the following harmful effectmay occur.

For example, when an excessive voltage of around 10 kV is applied to theprimary side of the LAN SPD, although a voltage is caused on thesecondary side of the LAN SPD by capacitive coupling of an isolationtransformer, the voltage caused on the secondary side is several kV(this voltage is larger than 1 kV, the withstand voltage of thecommunication device on the secondary side) and therefore acommunication device on the secondary side may be damaged. In otherwords, when an excessive voltage of 10 kV which the LAN SPD canwithstand is caused, the LAN SPD itself may not be damaged but thecommunication device on the secondary side may be damaged.

Therefore, it is requested to develop an LAN SPD that not only improvesthe withstand voltage of the LAN SPD simply but also does not damage thecommunication device on the secondary side even if the withstand voltageof the LAN SPD is improved.

(b) Regarding Chassis of LAN SPD

In a case where the withstand voltage performance of a LAN SPD isimproved up to around 13 kV and a lightning surge of around 20 kV overthe withstand voltage performance of the LAN SPD is applied to the LANSPD, an isolation transformer inside the LAN SPD is subjected tobreakdown (i.e. aerial discharge between the primary wire side and thesecondary wire side of the isolation transformer), thereby flowing outan excessive abnormal current from the primary wire side to thesecondary wire side of the isolation transformer. At this time, in achassis housing the LAN SPD, an inner pressure is rapidly increased bythermal expansion of air due to an abnormal current.

Here, for example, if the chassis of the LAN SPD having high pressureresistance performance is formed with an isolation member such as asynthetic resin as disclosed in the related art, it is not possible towithstand a rapid increase of chassis inner pressure at the time anabnormal current occurs, and the chassis may burst swiftly. Further,when the chassis bursts, fragments of the chassis fly off and thereforea risk to users is concerned.

Therefore, it can be considered to form the chassis with metal membersso as not to burst the chassis of the LAN SPD in a case where alightning surge over the withstand voltage performance of the LAN SPD isapplied to the LAN SPD. However, in the case of the chassis made ofmetal members, since the chassis is a conductor, it is necessary to makean isolation distance between the chassis and the SPD inner circuitlarger than that of the chassis made of isolation members, and therebythe LAN SPD itself becomes large and its cost increases.

Therefore, it is requested to develop a small LAN SPD that not onlyimproves the withstand voltage of the LAN SPD simply but also does notburst a chassis made of isolation members even if the withstand voltageof the LAN SPD is improved.

SUMMARY OF THE INVENTION

Therefore, to solve the foregoing problem of the related art, the firstobject of the present invention is to provide a communication lineisolator that does not damage a protection target device on thesecondary side even if the withstand voltage of the communication lineisolator is improved. Further, the second object of the presentinvention is to provide a small communication line isolator that doesnot burst a chassis made of isolation members even if the withstandvoltage of the communication line isolator is improved.

To achieve the first object, a communication line isolator of the firstinvention in the present invention is configured with an isolationtransformer including a primary winding connected to a communicationline and a secondary winding connected to a protection target devicethat performs communication with the communication line, and blocks anabnormal voltage entering from the communication line to protect theprotection target device. The protection target device has a function totransmit/receive a pulse signal to/from the communication line via apulse transformer and perform the communication, the pulse transformerincludes a primary winding connected to a side of the secondary windingof the isolation transformer and a secondary winding connected to aninner circuit side of the protection target device, and a medium tap inthe primary winding of the pulse transformer is grounded through aninner capacitance of the protection target device. Further, a parasiticcapacitance caused between the primary winding and the secondary windingin the isolation transformer and the inner capacitance are connected inseries, the abnormal voltage is divided by the parasitic capacitance andthe inner capacitance, and a capacitance value of the parasiticcapacitance is set such that a divided voltage of the inner capacitanceis equal to or below a predetermined value.

To achieve the second object, a communication line isolator of thesecond invention in the present invention is the communication lineisolator of the first invention, where: the isolation transformer isstored in a chassis formed with an isolation member (e.g. syntheticresin member) and is externally attached to the protection targetdevice; a pressure release hole is formed in an outer surface of thechassis; and the hole is sealed by a seal that can be opened when aninner pressure of the chassis increases.

According to the communication line isolator of the first invention, anisolator is formed using an isolation transformer having a parasiticcapacitance, such that: an abnormal voltage is divided by a combinedcapacitance in which the parasitic capacitance of the isolationtransformer and the inner capacitance in the protection target deviceare connected in series; and the divided voltage of the innercapacitance is equal to or below a predetermined value. Therefore, it ispossible to simply and accurately realize a high-withstand-voltageisolation-type isolator without applying an excessive divided voltage tothe protection target device side and causing a bad effect of damagingthe protection target device even if the isolator is subjected to a highwithstand voltage.

According to the communication line isolator of the second invention, anisolation transformer is stored in a chassis formed with an isolationmember, a pressure release hole is formed in the chassis to prevent aburst of the chassis at abnormal time (i.e. at the time an innerpressure increases), and the hole is sealed by a seal. Therefore, atnormal time, it is possible to prevent invasion of dust or damp from thehole into the chassis, and, at abnormal time (i.e. at the time the innerpressure increases), since the seal is removed or broken, it is possibleto let out the inner pressure and prevent a burst of the chassis.Therefore, it is possible to realize a communication line isolator inwhich, regardless of the chassis formed with the isolation member, thechassis is not burst even by an increase of the inner pressure.

The foregoing and other objects and new features of the presentinvention will be clarified when the description of the preferredembodiments below is read against the accompanying drawings. However,the drawings below are mainly provided for explanation and do not limitthe scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a rough circuit configuration diagram illustrating acommunication line isolator 10 according to a first embodiment of thepresent invention;

FIG. 1B is a rough circuit configuration diagram illustrating aprotection target device 40 according to the first embodiment of thepresent invention;

FIG. 2A and FIG. 2B are perspective views illustrating an outline of anisolator 10 in FIG. 1B;

FIG. 3A and FIG. 3B are rough configuration diagrams illustrating theinside of the isolator 10 in FIG. 2B;

FIG. 4 is a schematic waveform diagram illustrating the definition of animpulse waveform (10/700 μs);

FIG. 5A and FIG. 5B are schematic diagrams illustrating equivalentcircuits of capacities in FIG. 1A and FIG. 1B;

FIG. 6A, FIG. 6B and FIG. 6C are diagrams illustrating test verificationresults of capacitance in FIG. 5A and FIG. 5B;

FIG. 7 is a rough circuit configuration diagram illustrating acommunication line isolator 10A and a protection target device 40Aaccording to a second embodiment of the present invention;

FIG. 8A is a rough circuit configuration diagram illustrating acommunication line isolator 10B according to a third embodiment of thepresent invention;

FIG. 8B is a rough circuit configuration diagram illustrating aprotection target device 40B according to the third embodiment of thepresent invention;

FIG. 9 is a rough circuit configuration diagram illustrating acommunication line isolator 10C and a protection target device 40Caccording to a fourth embodiment of the present invention; and

FIG. 10 is a rough circuit configuration diagram illustrating maincomponents of a protection target device 40D according to a fifthembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment Configurationin First Embodiment

FIG. 1A is a rough circuit configuration diagram illustrating acommunication line isolator 10 according to a first embodiment of thepresent invention and FIG. 1B is a rough circuit configuration diagramillustrating a protection target device 40 according to the firstembodiment of the present invention.

As illustrated in FIG. 1A and FIG. 1B, the communication line isolator10 according to the first embodiment is an external isolation-type LANSPD (e.g. 1000Base-T SPD corresponding to gigabit Ethernet (registeredtrademark) standard) with high pressure resistance performance andwithout grounding. This isolator 10 is detachably connected between acommunication line (e.g. LAN cable formed with four twisted pairs ofwires or eight wires as a LAN line) 30 and a protection target device 40corresponding to a communication device.

The isolator 10 has a chassis 11 formed with an isolation member (e.g.synthetic resin member). A first connector (e.g. eight-position modularjack) 12 for input and output is attached to one side surface out of thefacing side surfaces of the chassis 11. An eight-position modular plug31 connected to a terminal unit of the LAN cable 30 is detachablyattached to this modular jack 12. In the chassis 11, four isolationtransformers 13 (=13-1 to 13-4) are stored. Each isolation transformer13 includes a primary winding 13 a and a secondary winding 13 b woundaround a core, each primary winding 13 a is connected to the modularjack 12 and each secondary winding 13 b is connected to one end of theLAN cable 14 formed with four twisted pairs of wires or eight wires.There is a parasitic capacitance Cs between the primary winding 13 a andthe secondary winding 13 b of each isolation transformer 13.

The other end of the LAN cable 14 is pulled out from the other sidesurface of the chassis 11 to the outside, and a second connector forinput and output (e.g. eight-position modular plug) 15 is connected tothe other end of the LAN cable 14. The modular plug 15 is detachablyattached to the protection target device 40.

The protection target device 40 includes, for example, an ONU, an HGWand a PC, and each component is stored in the chassis 41 as follows. Aconnector for input and output (e.g. eight-position modular jack) 42 isattached to an outer surface (e.g. side surface) of the chassis 41. Themodular jack 42 is detachably attached to the modular plug 15 on theside of the isolator 10. In the chassis 41, four pulse transformers 43(=43-1 to 43-4) for input and output, four resistances 44 (=44-1 to44-4) for impedance matching, a common inner capacitance Ci and acommunication controller (e.g. LAN controller) 50 as an inner circuit orthe like are stored.

Each pulse transformer 43 transmits a pulse signal for transmission andreception and includes a primary winding 43 a and a secondary winding 43b wound around a core, each primary winding 43 a is connected to themodular jack 42 and each secondary winding 43 b is connected to the LANcontroller 50. In the primary winding 43 a of each pulse transformer 43,a medium tap corresponding to a center tap is connected (i.e. grounded)to a frame grounding terminal of the chassis 41 via each resistance 44and the common inner capacitance Ci. For example, the resistance valueof each resistance 44 is 75Ω and the capacitance value of the commoninner capacitance Ci is 1000 pF.

The LAN controller 50 has a transmission and reception function totransmit and receive a transmission pulse signal and a reception pulsesignal in a switching manner, and, at the time of transmission,transmits transmission pulse signals generated in an non-illustratedprotection target device itself to the secondary windings 43 b of fourpulse transformers 43 (=43-1 to 43-4), and, at the time of reception,receives reception pulse signals transmitted from the secondary windings43 b of four pulse transformers 43 (=43-1 to 43-4) and gives them to thenon-illustrated protection target device itself.

FIG. 2A and FIG. 2B are perspective views illustrating an outline of theisolator 10 in FIG. 1B, where FIG. 2A is the overall perspective viewand FIG. 2B is a perspective view illustrating part of FIG. 2A. Further,FIG. 3A and FIG. 3B are rough configuration diagrams illustrating theinside of the isolator 10 in FIG. 2B, where FIG. 3A is a verticalcross-sectional view of the isolator 10 and FIG. 3B is a plan viewillustrating the inside of the isolator 10.

The chassis 11 storing, for example, the isolation transformers 13-1 to13-4 of the isolator 10 includes a lower member 21 having asubstantially square plate shape and an upper member 22 having asubstantially box shape in which the bottom surface to cover the surfaceof this lower member 21 is opened, and has a structure in which theupper member 22 is detachably fitted onto the lower member 21. A pluginsertion opening unit 23 is formed in one side surface out of thefacing side surfaces of the upper member 22. A cable pullout openingunit 24 is formed in the other side surface of the upper member 22 andthe side surface of the lower member 21 to contact to this other sidesurface.

In the upper member 22, there is designed a shielding unit 25 toseparate a modular jack storage space 22 a and an isolation transformerstorage space 22 b. Further, in an upper surface corresponding to anouter surface of the upper member 22, multiple pressure release holes 26are formed in a part corresponding to the isolation transformer storagespace 22 b in order to prevent a burst of the chassis 11 at abnormaltime (i.e. at the time an inner pressure increases). In the uppersurface of the upper member 22, a seal 27 is put on the parts in whichthe multiple holes 26 are formed, in a detachable or breakable manner,in order to prevent the degradation of withstand voltage performance dueto invasion of dust or damp from the holes 26 into the inside of thechassis. The seal 27 has functions of: sealing the multiple holes 26 atnormal time to prevent invasion of dust or damp; being removed or brokenat abnormal time (i.e. at the time the inner pressure increases) of thechassis 11 and opening these holes 26 to let out the inner pressure; andpreventing a burst of the chassis 11.

On the lower member 21, a printed circuit board 28 is attached. On theprinted circuit board 28, the modular jack 12 is fixed to a partcorresponding to the modular jack storage space 22 a, and, furthermore,four isolation transformers 13-1 to 13-4 are fixed at predeterminedintervals to a part corresponding to the isolation transformer storagespace 22 b. The terminal part of the LAN cable 14 is arranged betweentwo isolation transformers 13-1 and 13-2 and two isolation transformers13-3 and 13-4, and this LAN cable 14 is pulled out from the opening unit24 to the outside. The space between four isolation transformers 13-1 to13-4 and the modular jack 12 is shielded by the shielding unit 25. Theterminal unit of the LAN cable 14, the isolation transformers 13-1 to13-4 and the modular jack 12 are electrically connected to each other bya wiring pattern formed in the printed circuit board 28.

Setting of Pressure Resistance Performance in First Embodiment

FIG. 4 is a schematic waveform diagram illustrating the definition of animpulse waveform (10/700 μs).

In this FIG. 4, the horizontal axis represents time T (μs) and thevertical axis represents a voltage v (V). For example, in a lightningsurge of around 13 kV with an impulse waveform of 10/700 μs, thewaveform reaches the maximum voltage of 13 kV at the timing a time of 10μs passes from the rising start timing, and, after that, graduallyattenuates, and, at the timing a time of 700 μs passes from the risingstart timing, attenuates up to 7.5 kV, which is a half voltage of themaximum voltage of 13 kV.

FIG. 5A and FIG. 5B are schematic diagrams illustrating equivalentcircuits of the capacities in FIG. 1A and FIG. 1B, where FIG. 5A is acircuit diagram of the main components in FIG. 1A and FIG. 1B and FIG.5B is an equivalent circuit diagram of the capacitance in FIG. 5A.

The parasitic capacitance Cs of each isolation transformer 13 in theisolator 10 and the inner capacitance Ci in the protection target device40 are connected in series to the ground side. When an abnormal voltageVmax such as a lightning surge is applied from the LAN cable 30 to theisolator 10, this abnormal voltage Vmax is divided by the parasiticcapacitance Cs and the inner capacitance Ci, a divided voltage Vs isadded to the parasitic capacitance Cs and a divided voltage Vi is addedto the inner capacitance Ci.

A combined capacitance Ct of the parasitic capacitance Cs and the innercapacitance Ci connected in series is as follows:

Ct=(Cs×Ci)/(Cs+Ci)

An electric charge Qt of the whole series circuit, an electric charge Qsof the parasitic capacitance Cs part and an electric charge Qi of theinner capacitance Ci part are as follows:

Qt=Ct·Vmax

Qs=Cs·Vs

Qi=Ci·Vi

Since the electric charge Qt of the whole series circuit, the electriccharge Qs of the parasitic capacitance Cs and the electric charge Qi ofthe inner capacitance Ci are equal, Qt=Qs=Qi is established. From theabove equations, the divided voltages Vs and Vi are as followingequations (1) and (2).

$\begin{matrix}\begin{matrix}{{Vs} = {{Qs} \cdot \left( {1/{Cs}} \right)}} \\{= {\left( {{{Ct} \cdot V}\; \max} \right) \cdot \left( {1/{Cs}} \right)}} \\{= {{\left\lbrack {{Ci}/\left( {{Cs} + {Ci}} \right)} \right\rbrack \cdot V}\; \max}}\end{matrix} & (1) \\\begin{matrix}{{Vi} = {{Qi} \cdot \left( {1/{Ci}} \right)}} \\{= {\left( {{{Ct} \cdot V}\; \max} \right) \cdot \left( {1/{Ci}} \right)}} \\{= {{\left\lbrack {{Cs}/\left( {{Cs} + {Ci}} \right)} \right\rbrack \cdot V}\; \max}}\end{matrix} & (2)\end{matrix}$

Next, specific examples (a) to (c) and test verification results (d) ofthe specific examples will be explained.

(a) Specific Example 1

The withstand voltage of the isolator 10 is set to around 13 kV, thewithstand voltage of the protection target device 40 (i.e. innercapacitance Ci) is set to around 1.5 kV, and, furthermore, theelectrostatic capacitance value of the inner capacitance Ci is set to1000 pF.

For example, in a case where the electrostatic capacitance value of theparasitic capacitance Cs is set to 115 pF, when the abnormal voltageVmax (=13 kV) is applied, the divided voltage Vi of the innercapacitance Ci, which is represented by equation (2), changes toVi=[115/(115+1000)·13000≈1.34 kV. Accordingly, since the divided voltageVi of the inner capacitance Ci is 1.34 kV below the withstand voltage1.5 kV, the inner capacitance Ci is not damaged.

(b) Specific Example 2

The withstand voltage of the isolator 10 is set to around 13 kV, thewithstand voltage of the protection target device 40 (i.e. innercapacitance Ci) is set to around 1.5 kV, and, furthermore, theelectrostatic capacitance value of the inner capacitance Ci is set to1000 pF.

For example, in a case where the electrostatic capacitance value of theparasitic capacitance Cs is set to 10 pF, when the abnormal voltage Vmax(=13 kV) is applied, the divided voltage Vi of the inner capacitance Ci,which is represented by equation (2), changes toVi=[10/(10+1000)·13000≈129 kV. Accordingly, since the divided voltage Viof the inner capacitance Ci is 129 V below the withstand voltage 1.5 kV,the inner capacitance Ci is not damaged.

(c) Specific Example 3

Similar to the specific example 2, the withstand voltage of the isolator10 is set to around 13 kV, the withstand voltage of the protectiontarget device 40 (i.e. inner capacitance Ci) is set to around 1.5 kV,and, furthermore, the electrostatic capacitance value of the innercapacitance Ci is set to 1000 pF.

For example, in a case where the electrostatic capacitance value of theparasitic capacitance Cs is set to 10 pF, when the abnormal voltage Vmax(=5400 V) is applied, the divided voltage Vi of the inner capacitanceCi, which is represented by equation (2), changes toVi=[10/(10+1000)·5400≈53V. Accordingly, since the divided voltage Vi ofthe inner capacitance Ci is 53 V below the withstand voltage 1.5 kV, theinner capacitance Ci is not damaged.

(d) Test Verification Results of Specific Examples

FIG. 6A, FIG. 6B and FIG. 6C are diagrams illustrating test verificationresults of the capacities in FIG. 5A and FIG. 5B, where FIG. 6A is aschematic diagram illustrating a test apparatus and, furthermore, FIG.6B and FIG. 6C are waveform diagrams of the test verification results.

In FIG. 6A, similar to the specific example 3, using a high-frequencygenerator IG, 5400 V is applied between both ends of the series circuitformed with the parasitic capacitance Cs (=10 pF) and the innercapacitance Ci (=1000 pF), a voltage between both nodes CH1 and CH2 ofthe parasitic capacitance Cs is observed by an oscilloscope OSC and thewaveform diagram of FIG. 6B is acquired. The voltage of the node CH1 isan applied voltage and the voltage of the node CH2 is a residual voltageof the inner capacitance Ci.

As clear from the waveform diagram of FIG. 6B, the divided voltage Vi ofthe inner capacitance Ci is 128 V. Therefore, the lightning surgeattenuation rate is 128/5400=2.37/100=2.37%.

According to the test verification results, the divided voltage Vi ofthe inner capacitance Ci is 128 V, which is larger by 53 V than thetheoretical value explained in the specific example 3. This may bebecause there is an electrostatic capacitance value of around 10 pFbetween the ground, the oscilloscope OSC and the node CH1 and betweenthe ground, the oscilloscope OSC and the node CH2. In any event, sincethe divided voltage Vi of the inner capacitance Ci is below thewithstand voltage 1.5 kV, the inner capacitance Ci is not damaged.

In view of these, if 13000 V (=13 kV) is applied between both ends ofthe series circuit formed with the parasitic capacitance Cs (=10 pF) andthe inner capacitance Ci (=1000 pF), a voltage (=divided voltage Vi)appearing on the node CH2 of the inner capacitance Ci is estimated to bearound 308 V based on the following equation, and, when this estimationwaveform diagram is shown, it is as shown in FIG. 6C.

13000 V×0.0237=308 V

This estimated divided voltage Vi of 308 V is larger than 129 V which isthe theoretical value explained in the specific example 2. This may bebecause, as described above, there is an electrostatic capacitance valueof around 10 pF between the ground, the oscilloscope OSC and the nodeCH1 and between the ground, the oscilloscope OSC and the node CH2. Inany event, since the divided voltage Vi of the inner capacitance Ci isbelow the withstand voltage 1.5 kV, the inner capacitance Ci is notdamaged.

A feature of the first embodiment is to configure the isolator 10 usingfour isolation transformers 13-1 to 13-4 each having the parasiticcapacitance Cs, such that the abnormal voltage Vmax is divided by thecombined capacitance Ct and the divided voltage Vi of the innercapacitance Ci is equal to or below a predetermined value (e.g. equal toor below a withstand voltage of 1.5 V), where the combined capacitanceCt is formed by connecting the parasitic capacitance Cs of eachisolation transformer 13 in the isolator 10 and the inner capacitance Ciin the protection target device 40 in series.

Operation in First Embodiment

An explanation will be given to an operation (I) at normal time and anoperation (II) at abnormal time in the isolator 10 and the protectiontarget device 40 illustrated in FIGS. 1A, 1B, 2A, 2B, 3A and 3B.

(I) Operation at Normal Time

In FIG. 1A and FIG. 1B, at the time of transmission in the protectiontarget device 40, a transmission pulse signal output from the LANcontroller 50 in the protection target device 40 is transmitted to themodular jack 42 via four pulse transformers 43-1 to 43-4. Thetransmission pulse signal transmitted to the modular jack 42 istransmitted to the modular jack 12 via the modular plug 15, the LANcable 14 and four isolation transformers 13-1 to 13-4 on the side of theisolator 10. The transmission pulse signal transmitted to the modularjack 12 is transmitted to the LAN cable 30 via the modular plug 31.

Also, at the time of reception in the protection target device 40, areception pulse signal transmitted from the LAN cable 30 is input in themodular jack 12 on the side of the isolator 10 via the modular plug 31.The reception pulse signal input in the modular jack 12 is transmittedto the modular jack 42 on the side of the protection target device 40via four isolation transformers 13-1 to 13-4, the LAN cable 14 and themodular plug 15 in the isolator 10. The reception pulse signaltransmitted to the modular jack 42 is received in the LAN controller 50via four pulse transformers 43-1 to 43-4 in the protection target device40.

(II) Operation at Abnormal Time

For example, the withstand voltage of the isolator 10 is set to around13 kV. Further, the withstand voltage of the protection target device 40(i.e. withstand voltage of the inner capacitance Ci) is set to around1.5 kV and the electrostatic capacitance value is set to 1000 pF, and,in response to this, the electrostatic capacitance value of theparasitic capacitance Cs is set to 115 pF or 10 pF.

In a case where the abnormal voltage Vmax (e.g. 13 kV) such as alightning surge enters from the LAN cable 30 into the side of theisolator 10, the current of the abnormal voltage Vmax passes from theprimary winding 13 a to the secondary winding 13 b through capacitancecoupling by the parasitic capacitance Cs of each isolation transformer13 in the isolator 10.

The current of the abnormal voltage Vmax passing through the parasiticcapacitance Cs of each isolation transformer 13 enters the protectiontarget device 40 via the LAN cable 14, the modular plug 15 and themodular jack 42. The current of the abnormal voltage Vmax having enteredthe protection target device 40 is released to the ground side via themedium tap of the first winding 43 a in each pulse transformer 43, eachresistance 44 and the inner capacitance Ci. At this time, since theparasitic capacitance Cs and the inner capacitance Ci are connected inseries to the ground side, the abnormal voltage Vmax is divided by theparasitic capacitance Cs and the inner capacitance Ci, the dividedvoltage Vs is added to the parasitic capacitance Cs and the dividedvoltage Vi is added to the inner capacitance Ci.

The electrostatic capacitance value of the inner capacitance Ci is setto 1000 pF, and, in response to this, the electrostatic capacitancevalue of the parasitic capacitance Cs is set to 115 pF or 10 pF.Therefore, since the divided voltage Vi of the inner capacitance Ci isbelow 1.5 kV, the inner capacitance Ci is not damaged.

Meanwhile, with respect to the withstand voltage performance systemvalue of 13 kV in the isolator 10, if the abnormal voltage Vmax (e.g.1.2/50 μs and 15 kV) such as a lightning surge enters from the LAN cable30 into the side of the isolator 10, there are risks that: the isolationtransformers 13 (=13-1 to 13-4) in the isolator 10 are subjected tobreakdown; an excessive abnormal current (e.g. abnormal current at thetime of this withstand voltage breakdown is 8/20 μs and 7.5 kA) is letout from the side of the primary winding 13 a of these isolationtransformers 13 to the side of the secondary winding 13 b; and theprotection target device 40 is broken. If such an abnormal current isflowed, in the chassis 11 storing the isolation transformers 13, thermalexpansion of air due to the abnormal current causes a rapid increase ofan inner pressure, the seal 27 sealing the hole 26 of the chassis 11 isremoved or broken and the inner pressure is discharged from the hole 26to the outside. By this means, it is possible to prevent a burst of thechassis 11.

Also, to protect the breakdown of the isolator 10 from the abnormalvoltage Vmax equal to or above the withstand voltage as described above,the withstand voltage performance system value of the isolator 10 may beset to a value equal to or above 13 kV according to the predictableabnormal voltage Vmax.

Effect of First Embodiment

According to the first embodiment, following effects (i) and (ii) areprovided.

(i) According to the first embodiment, the isolator 10 is formed usingfour isolation transformers 13-1 to 13-4 each having the parasiticcapacitance Cs, such that the abnormal voltage Vmax is divided by thecombined capacitance Ct and the divided voltage Vi of the innercapacitance Ci is equal to or below a predetermined value (e.g. equal toor below a withstand voltage of 1.5 V), where the combined capacitanceCt is formed by connecting the parasitic capacitance Cs of eachisolation transformer 13 in the isolator 10 and the inner capacitance Ciin the protection target device 40 in series. Therefore, it is possibleto simply and accurately realize the high-withstand-voltageisolation-type isolator 10, without applying the excessive dividedvoltage Vi to the side of the protection target device 40 and causing abad effect of damaging the protection target device 40 even if theisolator 10 is subjected to a high withstand voltage.

(ii) To prevent a burst of the chassis 11 at abnormal time (i.e. at thetime the inner pressure increases), the pressure release hole 26 isformed in this chassis 11 and this hole 26 is sealed by the seal 27.Therefore, at normal time, it is possible to prevent invasion of dust ordamp from the hole 26 into the chassis 11, and, at abnormal time (i.e.at the time the inner pressure increases), since the seal 27 is removedor broken, it is possible to let out the inner pressure and prevent aburst of the chassis 11. Therefore, it is possible to realize theisolator 10 in which, regardless of the chassis 11 made of syntheticresin members, the chassis 11 is not burst even by an increase of theinner pressure.

Second Embodiment Configuration in Second Embodiment

FIG. 7 is a rough circuit configuration diagram illustrating thecommunication line isolator 10A and the protection target device 40Aaccording to the second embodiment of the present invention. In thisFIG. 7, the same reference numerals are assigned to the same componentsas the components in FIGS. 1A and 1B illustrating the first embodiment.

The communication line isolator 10A according to the second embodimentis a built-in isolation-type LAN SPD (e.g. 1000Base-T SPD correspondingto gigabit Ethernet (registered trademark) standard) with high pressureresistance performance and without grounding. In this isolator 10A, fourisolation transformers 13 (=13-1 to 13-4) forming the communication lineisolator 10A are set in the chassis 41 storing the protection targetdevice 40A corresponding to a communication device.

The eight-position modular jack 42 is attached to the side surface ofthe chassis 41 storing the protection target device 40A. Theeight-position modular plug 31 connected to the LAN cable 30 isdetachably attached to the modular jack 42. In the chassis 41, fourinput-output isolation transformers 13 (=13-1 to 13-4) forming thecommunication line isolator 10A, four input-output pulse transformers 43(=43-1 to 43-4), four resistances 44 (=44-1 to 44-4) for impedancematching, the common inner capacitance Ci and the LAN controller 50 orthe like are stored.

As illustrated in FIG. 1A, each isolation transformer 13 (=13-1 to 13-4)includes the primary winding 13 a and the secondary winding 13 b, whereeach primary winding 13 a is connected to the modular jack 42 and eachsecondary winding 13 b is connected to the primary winding 43 a of eachpulse transformer 43. Similar to FIG. 1B, the medium tap in the primarywinding 43 a of each pulse transformer 43 is grounded to a framegrounding terminal of the chassis 41 via each resistance 44 and thecommon inner capacitance Ci. For example, the resistance value of eachresistance 44 is 75Ω and the capacitance value of the common innercapacitance Ci is 1000 pF. Further, the secondary winding 43 b of eachpulse transformer 43 is connected to the LAN controller 50.

Setting of Withstand Voltage Performance in Second Embodiment

The setting of withstand voltage performance in the second embodiment isset in the same way as in the first embodiment.

Operation in Second Embodiment

An explanation will be given to an operation (I) at normal time and anoperation (II) at abnormal time in the second embodiment.

(I) Operation at Normal Time

Similar to the first embodiment, at the time of transmission in theprotection target device 40A, a transmission pulse signal output fromthe LAN controller 50 is transmitted to the modular jack 42 via fourpulse transformers 43-1 to 43-4 and four isolation transformers 13-1 to13-4. The transmission pulse signal transmitted to the modular jack 42is transmitted to the LAN cable 30 via the modular plug 31.

Also, at the time of reception in the protection target device 40A, areception pulse signal transmitted from the LAN cable 30 is input in themodular jack 12 via the modular plug 31. The reception pulse signalinput in the modular jack 42 is received in the LAN controller 50 viafour isolation transformers 13-1 to 13-4 and four pulse transformers43-1 to 43-4.

(II) Operation at Abnormal Time

Similar to the first embodiment, for example, the withstand voltage ofthe isolators 13 (=13-1 to 13-4) is set to around 13 kV. Further, thewithstand voltage of the inner capacitance Ci is set to around 1.5 kVand the electrostatic capacitance value is set to 1000 pF, and, inresponse to this, the electrostatic capacitance value of the parasiticcapacitance Cs is set to 115 pF or 10 pF.

In a case where the abnormal voltage Vmax (e.g. 13 kV) such as alightning surge enters from the LAN cable 30, similar to the firstembodiment, the current of the abnormal voltage Vmax passes from theprimary winding 13 a to the secondary winding 13 b through capacitancecoupling by the parasitic capacitance Cs of each isolation transformer13. The current of the abnormal voltage Vmax passing through theparasitic capacitance Cs of each isolation transformer 13 is released tothe ground side via the medium tap of the first winding 43 a in eachpulse transformer 43, each resistance 44 and the inner capacitance Ci.At this time, since the parasitic capacitance Cs and the innercapacitance Ci are connected in series to the ground side, the abnormalvoltage Vmax is divided by the parasitic capacitance Cs and the innercapacitance Ci, the divided voltage Vs is added to the parasiticcapacitance Cs and the divided voltage Vi is added to the innercapacitance Ci.

The electrostatic capacitance value of the inner capacitance Ci is setto 1000 pF, and, in response to this, the electrostatic capacitancevalue of the parasitic capacitance Cs is set to 115 pF or 10 pF.Therefore, since the divided voltage Vi of the inner capacitance Ci isbelow 1.5 kV, the inner capacitance Ci is not damaged.

Meanwhile, with respect to the withstand voltage performance systemvalue of 13 kV in the isolator 10, if the abnormal voltage Vmax (e.g.1.2/50 μs and 15 kV) such as a lightning surge enters from the LAN cable30, there are risks that: the isolation transformers 13 (=13-1 to 13-4)are subjected to breakdown; an excessive abnormal current is let outfrom the side of the primary winding 13 a of these isolationtransformers 13 to the side of the secondary winding 13 b; and theprotection target device 40A is broken. To protect the breakdown of theprotection target device 40A from the abnormal voltage Vmax equal to orabove the withstand voltage as described above, the withstand voltageperformance system value of the isolation transformer 13 may be set to avalue equal to or above 13 kV according to the predictable abnormalvoltage Vmax. Also, in a case where there is a risk that the chassis 41is burst by thermal expansion of air in the chassis 41 due to such anabnormal current, for example, similar to the first embodiment, it maybe possible to adopt a preventative measure of forming the pressurerelease hole 26 in the side surface of the chassis 41 and sealing thishole 26 by the seal 27.

Effect of Second Embodiment

According to the second embodiment, the similar effect to the effect (i)in the first embodiment is provided. Further, four isolationtransformers 13 (=13-1 to 13-4) forming the communication line isolator10A are incorporated in the chassis 41 on the side of the protectiontarget device 40A. Therefore, by omitting components such as the LANcable 14 and the modular plug 15 illustrated in FIG. 1A, it is possibleto downsize the whole device.

Third Embodiment Configuration in Third Embodiment

FIG. 8A is a rough circuit configuration diagram illustrating acommunication line isolator 10B according to the third embodiment of thepresent invention and FIG. 8B is a rough circuit configuration diagramillustrating the protection target device 40B according to the thirdembodiment. In these FIG. 8A and FIG. 8B, the same reference numeralsare assigned to the same components as the components in FIGS. 1A and 1Billustrating the first embodiment.

The communication line isolator 10B according to the third embodiment isan external isolation-type LAN SPD (e.g. 10Base-T or 100Base-TX SPDcorresponding to Ethernet (registered trademark) standard) with highpressure resistance performance and without grounding. This isolator 10Bis detachably connected between the LAN cable 30 and the protectiontarget device 40B corresponding to a communication device.

Similar to the first embodiment, the isolator 10B has the chassis 11formed with an isolation member (e.g. synthetic resin member), and themodular jack 12 is attached to the side surface of this chassis 11. Inthe chassis 11, the transmission isolation transformer 13-1 and thereception isolation transformer 13-2 are stored. The isolationtransformers 13-1 and 13-2 each include the primary winding 13 a and thesecondary winding 13 b, where each primary winding 13 a is connected tothe modular jack 12 and each secondary winding 13 b is connected to theone end of the LAN cable 14. Similar to the first embodiment, there isthe parasitic capacitance Cs between the primary winding 13 a and thesecondary winding 13 b of the isolation transformers 13-1 and 13-2.Similar to the first embodiment, the other end of the LAN cable 14 ispulled out from the side surface of the chassis 11 to the outside, andthe modular plug 15 is connected to the other end of the LAN cable 14.The modular plug 15 is detachably attached to the protection targetdevice 40B.

Similar to the first embodiment, the protection target device 40Bincludes, for example, an ONU, an HGW and a PC, and each component isstored in the chassis 41 as follows. The modular jack 42 is attached tothe side surface of the chassis 41. In the chassis 41, the transmissionpulse transformer 43-1, the reception pulse transformer 43-2, sixresistances 44-1, 44-2 and 45-1 to 45-4 for impedance matching, thecommon inner capacitance Ci and the LAN controller 50B as an innercircuit or the like are stored.

Similar to the first embodiment, the pulse transformers 43-1 and 43-2each include the primary winding 43 a and the secondary winding 43 b,each primary winding 43 a is connected to the modular jack 42 and eachsecondary winding 43 b is connected to the LAN controller 50B. Themedium tap in the primary winding 43 a of each of the pulse transformers43-1 and 43-2 is grounded to a frame grounding terminal of the chassis41 via the resistances 44-1 and 44-2 and the common inner capacitanceCi. Four available electrodes of the modular jack 42 are grounded to theframe grounding terminal of the chassis 41 via the resistances 45-1 to45-4 and the common inner capacitance Ci. Similar to the firstembodiment, for example, the resistance value of each of the resistances44-1, 44-2 and 45-1 to 45-4 is 75Ω and the capacitance value of thecommon inner capacitance Ci is 1000 pF.

The LAN controller 50B has a transmission and reception function totransmit and receive a transmission pulse signal and reception pulsesignal in a switching manner. This LAN controller 50B is configured to:at the time of transmission, transmit a transmission pulse signalgenerated in a non-illustrated protection target device itself to thetransmission pulse transformer 43-1; and, at the time of reception,receive a reception pulse signal transmitted from the reception pulsetransformer 43-2 and give it to the non-illustrated protection targetdevice itself.

Other configurations are similar to the first embodiment.

Setting of Withstand Voltage Performance in Third Embodiment

The setting of withstand voltage performance in the third embodiment isset in a similar way to the first embodiment.

Operation in Third Embodiment

An explanation will be given to an operation (I) at normal time and anoperation (II) at abnormal time in the third embodiment.

(I) Operation at Normal Time

At the time of transmission in the protection target device 40B, atransmission pulse signal output from the LAN controller 50B istransmitted to the modular jack 12 via the transmission pulsetransformer 43-1, the modular jack 42, the modular plug 15, the LANcable 14 and the transmission isolation transformer 13-1. Thetransmission pulse signal transmitted from the modular jack 12 istransmitted to the LAN cable 30 via the modular plug 31.

Also, at the time of reception in the protection target device 40B, areception pulse signal transmitted from the LAN cable 30 is received inthe LAN controller 50B via the modular plug 31, the modular jack 12, thereception isolation transformer 13-2, the LAN cable 14, the modular plug15, the modular jack 42 and the reception pulse transformer 43-2.

(II) Operation at Abnormal Time

Similar to the first embodiment, for example, the withstand voltage ofthe isolation transformers 13-1 and 13-2 is set to around 13 kV.Further, the withstand voltage of the inner capacitance Ci is set toaround 1.5 kV and the electrostatic capacitance value is set to 1000 pF,and, in response to this, the electrostatic capacitance value of theparasitic capacitance Cs is set to 115 pF or 10 pF.

In a case where the abnormal voltage Vmax (e.g. 13 kV) such as alightning surge enters from the LAN cable 30, similar to the firstembodiment, the current of the abnormal voltage Vmax passes from theprimary winding 13 a to the secondary winding 13 b through capacitancecoupling by the parasitic capacitance Cs of the isolation transformers13-1 and 13-2. The current of the abnormal voltage Vmax passing throughthe parasitic capacitance Cs of the isolation transformers 13-1 and 13-2is released to the ground side via the medium tap of the first winding43 a in the pulse transformers 43-1 and 43-2, the resistances 44-1 and44-2 and the inner capacitance Ci. At this time, since the parasiticcapacitance Cs and the inner capacitance Ci are connected in series tothe ground side, the abnormal voltage Vmax is divided by the parasiticcapacitance Cs and the inner capacitance Ci, the divided voltage Vs isadded to the parasitic capacitance Cs and the divided voltage Vi isadded to the inner capacitance Ci.

The electrostatic capacitance value of the inner capacitance Ci is setto 1000 pF, and, in response to this, the electrostatic capacitancevalue of the parasitic capacitance Cs is set to 115 pF or 10 pF.Therefore, since the divided voltage Vi of the inner capacitance Ci isbelow 1.5 kV, the inner capacitance Ci is not damaged.

Meanwhile, with respect to the withstand voltage performance systemvalue of 13 kV in the isolator 10B, if the abnormal voltage Vmax (e.g.1.2/50 μs and 15 kV) such as a lightning surge enters from the LAN cable30, similar to the first embodiment, there are risks that: the isolationtransformers 13-1 and 13-2 are subjected to breakdown; an excessiveabnormal current is let out from the side of the primary winding 13 a ofthese isolation transformers 13-1 and 13-2 to the side of the secondarywinding 13 b; and the protection target device 40B is broken. If such anabnormal current is flowed, in the chassis 11 storing the isolationtransformers 13-1 and 13-2, thermal expansion of air due to the abnormalcurrent causes a rapid increase of an inner pressure, the seal 27sealing the hole 26 of the chassis 11 is removed or broken and the innerpressure is discharged from the hole 26 to the outside. By this means,it is possible to prevent a burst of the chassis 11.

Also, similar to the first embodiment, to protect the breakdown of theisolator 10B from the abnormal voltage Vmax equal to or above thewithstand voltage as described above, the withstand voltage performancesystem value of the isolator 10B may be set to a value equal to or above13 kV according to the predictable abnormal voltage Vmax.

Effect of Third Embodiment

According to the third embodiment, the similar effects to the effects(i) and (ii) in the first embodiment are provided.

Fourth Embodiment Configuration in Fourth Embodiment

FIG. 9 is a rough circuit configuration diagram illustrating thecommunication line isolator 10C and the protection target device 40Caccording to the fourth embodiment of the present invention. In thisFIG. 9, the same reference numerals are assigned to the same componentsas the components in FIGS. 8A and 8B illustrating the third embodiment.

The communication line isolator 10C according to the fourth embodimentis a built-in isolation-type LAN SPD (e.g. 10Base-T or 100Base-TX SPDcorresponding to Ethernet (registered trademark) standard) with highpressure resistance performance and without grounding. In this isolator10C, the transmission isolation transformer 13-1 and the receptionisolation transformer 13-2 forming the communication line isolator 10Care set in the chassis 41 storing the protection target device 40Ccorresponding to a communication device.

The modular jack 42 is attached to the side surface of the chassis 41storing the protection target device 40C. The modular plug 31 connectedto the LAN cable 30 is detachably attached to the modular jack 42. Inthe chassis 41, the transmission isolation transformer 13-1 and thereception isolation transformer 13-2 forming the communication lineisolator 10C, the transmission pulse transformer 43-1 and the receptionpulse transformer 43-2, six resistances 44-1, 44-2 and 45-1 to 45-4 forimpedance matching, the common inner capacitance Ci and the LANcontroller 50B as an inner circuit or the like are stored.

As illustrated in FIG. 8A, the isolation transformers 13-1 and 13-2 eachinclude the primary winding 13 a and the secondary winding 13 b, whereeach primary winding 13 a is connected to the modular jack 42 and eachsecondary winding 13 b is connected to the primary winding 43 a of eachof the pulse transformers 43-1 and 43-2. Similar to FIG. 8B, the mediumtap in the primary winding 43 a of each of the pulse transformers 43-1and 43-2 is grounded to a frame grounding terminal of the chassis 41 viathe resistances 44-1 and 44-2 and the common inner capacitance Ci.Similar to the third embodiment, four available electrodes of themodular jack 42 are grounded to the frame grounding terminal of thechassis 41 via the resistances 45-1 to 45-4 and the common innercapacitance Ci. Similar to the third embodiment, for example, theresistance value of each of the resistances 44-1, 44-2 and 45-1 to 45-4is 75Ω and the capacitance value of the common inner capacitance Ci is1000 pF. Further, the secondary winding 43 b of the pulse transformers43-1 and 43-2 is connected to the LAN controller 50B.

Setting of Withstand Voltage Performance in Fourth Embodiment

The setting of withstand voltage performance in the fourth embodiment isset in the same way as the third embodiment.

Operation in Fourth Embodiment

An operation at normal time and an operation at abnormal time in thefourth embodiment are substantially the same as the third embodiment.

Also, as the operation at abnormal time, similar to the secondembodiment, with respect to a withstand voltage performance system valueof 13 kV in the isolation transformers 13-1 and 13-2, if the abnormalvoltage Vmax (e.g. 1.2/50 μs and 15 kV) such as a lightning surge entersfrom the LAN cable 30, there are risks that: the isolation transformers13-1 and 13-2 are subjected to breakdown; an excessive abnormal currentis let out from the side of the primary winding 13 a of these isolationtransformers 13-1 and 13-2 to the side of the secondary winding 13 b;and the protection target device 40C is broken. To protect the breakdownof the protection target device 40C from the abnormal voltage Vmax equalto or above the withstand voltage as described above, the withstandvoltage performance system value of the isolation transformers 13-1 and13-2 may be set to a value equal to or above 13 kV according to thepredictable abnormal voltage Vmax. Also, in a case where there is a riskthat the chassis 41 is burst by thermal expansion of air in the chassis41 due to such an abnormal current, for example, similar to the firstembodiment, it may be possible to adopt a preventative measure offorming the pressure release hole 26 in the side surface of the chassis41 and sealing this hole 26 by the seal 27.

Result of Fourth Embodiment

According to the fourth embodiment, the similar effect to the effect (i)in the first embodiment is provided. Further, two isolation transformers13-1 and 13-2 forming the communication line isolator 10C areincorporated in the chassis 41 on the side of the protection targetdevice 40C. Therefore, by omitting components such as the LAN cable 14and the modular plug 15 illustrated in FIG. 8A, it is possible todownsize the whole device.

Fifth Embodiment

FIG. 10 is a rough circuit configuration diagram illustrating maincomponents of a protection target device 40D according to the fifthembodiment of the present invention. In this FIG. 10, the same referencenumerals are assigned to the same components as the components in FIGS.1A and 1B illustrating the first embodiment.

In the protection target device 40D according to the fifth embodiment, acommon mode choke coil 46 is connected in series between the modularjack 42 and the primary winding 43 a of the pulse transformer 43-1.Similarly, although it is not illustrated, the common mode choke coil 46is connected in series between the modular jack 42 and the primarywinding 43 a of each of the other pulse transformers 43-2 to 43-4. Bysetting such the common mode choke coil 46, a common mode noise issuppressed and the performance of the protection target device 40D isimproved.

Also, if the above modular jack 42 is also provided in the secondembodiment, the third embodiment and the fourth embodiment, theperformance of the protection target devices 40A, 40B and 40C isimproved.

Other Variation Example of First to Fifth Embodiments

The present invention is not limited to the above first to fifthembodiments and various utility forms or variations are possible. Asthese utility forms or variation examples, for example, there arefollowing (a) to (c).

(a) The communication line isolators 10, 10A, 10B and 10C and theprotection target devices 40, 40A, 40B, 40C and 40D according to thefirst to fifth embodiments may be changed to other circuits orconfigurations than those in the drawings. For example, in FIG. 1A orFIG. 8A, it may be changed to a configuration in which the LAN cable 14is omitted and the modular plug 15 is attached to the side surface ofthe chassis 11. Also, the resistances 44-1 to 44-3 and 45-1 to 45-4 inthe protection target devices 40, 40A, 40B, 40C and 40D may be omitted.Further, in FIG. 3A and FIG. 3B, although the isolation transformer 13is packaged every line like the isolation transformers 13-1 to 13-4,they may be collectively packaged for all lines.

(b) A synthetic resin member forming the chassis 11 may be changed toother isolation members such as ceramics.

(c) In the first to fifth embodiments, although an explanation is givento the communication line isolators 10, 10A, 10B and 10 C with respectto the LAN cable 30, the communication line isolator of the presentinvention is applicable to other communication lines than the LAN cable30.

What is claimed is:
 1. A communication line isolator that is configuredwith an isolation transformer including a primary winding connected to acommunication line and a secondary winding connected to a protectiontarget device that performs communication with the communication line,and blocks an abnormal voltage entering from the communication line toprotect the protection target device, wherein: the protection targetdevice has a function to transmit/receive a pulse signal to/from thecommunication line via a pulse transformer and perform thecommunication, the pulse transformer includes a primary windingconnected to a side of the secondary winding of the isolationtransformer and a secondary winding connected to an inner circuit sideof the protection target device, and a medium tap in the primary windingof the pulse transformer is grounded through an inner capacitance of theprotection target device; and a parasitic capacitance caused between theprimary winding and the secondary winding in the isolation transformerand the inner capacitance are connected in series, the abnormal voltageis divided by the parasitic capacitance and the inner capacitance, and acapacitance value of the parasitic capacitance is set such that adivided voltage of the inner capacitance is equal to or below apredetermined value.
 2. The communication line isolator according toclaim 1, wherein: the isolation transformer is stored in a chassisformed with an isolation member and is externally attached to theprotection target device; a pressure release hole is formed in an outersurface of the chassis; and the hole is sealed by a seal that can beopened when an inner pressure of the chassis increases.
 3. Thecommunication line isolator according to claim 2, wherein the isolationmember is a synthetic resin member.
 4. The communication line isolatoraccording to claim 1, wherein the isolation transformer is incorporatedin the protection target device.
 5. The communication line isolatoraccording to claim 1, wherein the medium tap is grounded through aresistance for impedance matching and the inner capacitance.
 6. Thecommunication line isolator according to claim 1, wherein the primarywinding of the pulse transformer is connected to the side of thesecondary winding of the isolation transformer through a common modechoke coil.
 7. The communication line isolator according to claim 1,wherein the communication line is a local area network.
 8. Thecommunication line isolator according to claim 5, wherein the primarywinding of the pulse transformer is connected to the side of thesecondary winding of the isolation transformer through a common modechoke coil.
 9. The communication line isolator according to claim 2,further comprising: the isolation transformer; a first input/outputconnector that is connected to the primary winding of the isolationtransformer and detachably connected to the communication line; and asecond input/output connector that is connected to the secondary windingof the isolation transformer and detachably connected to the protectiontarget device, wherein: the isolation transformer and the firstconnector are stored in the chassis; and the second connector is pulledout from the chassis by a cable.
 10. The communication line isolatoraccording to claim 3, further comprising: the isolation transformer; afirst input/output connector that is connected to the primary winding ofthe isolation transformer and detachably connected to the communicationline; and a second input/output connector that is connected to thesecondary winding of the isolation transformer and detachably connectedto the protection target device, wherein: the isolation transformer andthe first connector are stored in the chassis; and the second connectoris pulled out from the chassis by a cable.
 11. The communication lineisolator according to claim 3, further comprising: the isolationtransformer; a first input/output connector that is connected to theprimary winding of the isolation transformer and detachably connected tothe communication line; and a second input/output connector that isconnected to the secondary winding of the isolation transformer anddetachably connected to the protection target device, wherein theisolation transformer, the first connector and the second connector arestored in the chassis.